Iron and Biogeochemical Cycles. Redfield Ratio C:N:P (“macro-nutrients”) 106:16 :1 ( Redfield, 1958) Could there be other essential micro- nutrients?

Presentation on theme: "Iron and Biogeochemical Cycles. Redfield Ratio C:N:P (“macro-nutrients”) 106:16 :1 ( Redfield, 1958) Could there be other essential micro- nutrients?"— Presentation transcript:

1 Iron and Biogeochemical Cycles

2 Redfield Ratio C:N:P (“macro-nutrients”) 106:16 :1 ( Redfield, 1958) Could there be other essential micro- nutrients? -Trace metals such as Fe, Mn, Zn, Cu, Co are important! - Fe needed in enzymes crucial for photosynthesis, nitrate assimilation, and N2 fixation.

3 High Nutrient, Low (Medium) Chlorophyll Regions (HNLC) Why aren’t the nutrients being completely utilized by phytoplankton? PhosphorousChlorophyll Conkright et al., 1994 µm SeaWiFs

4 Rate of Fe dust deposition from atmosphere High-nutrient, low- chlorophyll (HNLC) regions of the ocean

5 Hypotheses to explain HNLC regions Light Grazing Micronutrient limitation

6 Galapagos Islands from SeaWifs satellite Islands Red = most chlorophyll

7 In situ Langrangian Fe enrichment studies Idea: 1.Add dissolved Fe to a patch of an HNLC region perhaps 10km x 10km. 2.Keep track of the patch using a passive, conservative tracer (SF 6 ) 3.Measure chemical and biological parameters relating to primary productivity both in and out of the patch, over a period of days-weeks. 4.This has now been done several times in each of the major HNLC regions. 5.See papers in Science and Nature by Boyd and Coale (project leaders).

8 Changes in Chlorophyll Chemical Oceanography

9 Cruise Track Chemical Oceanography

10 In situ Fertilization experiments: Is iron limiting? e.g. Australia Positive results visible from space!

11 SF 6 Fe Chl NO 3 pCO 2 123456Day

12 ‘Dissolved’ Iron distribution Why are there so few measurements? - Difficult to sample cleanly and to measure accurately. Surface1000 m

13 Iron Profile What controls the distribution (vertically and horizontally) of Iron? Iron has a profile between a typical scavenged metal and nutrient Scavenged Metal (Al) Nutrient-type metal

14 Sources of Iron Riverine Continental Shelves Dust Hydrothermal vents

15 Riverine [Fe’] decreases rapidly as river water mixes with seawater This is due to scavenging of Fe by particles and coagulation of particles at high ionic strength We can conclude that rivers are not an important source for the open ocean - only coastal (Fe has very short residence time) Boyle et al. (1977)

16 Scavenging: Iron sink Iron lost to the ocean by scavenging – the process of sticking onto particles Rate of scavenging not well-known loss=-ksc[Fe’][P] Fe’ = sum of inorganic dissolved Fe species

17 Continental Shelves 1. Resuspension of sediments can release Fe 2. When organic matter decomposes, Fe can diffuse or be bio-irrigated into the water column C 106 H 263 O 110 N 16 P 1 Fe.0001 +138O 2 =106C O 2 +16NO - 3 +H 2 PO - 4 +0.0001Fe(OH) 2 Estimate global flux of 0.2-9 x 10 10 mol y -1 Is this Fe upwelled to the surface before being scavenged? Active area of research Results from sediment core and flux chamber experiment (Elrod et al., 2004)

18 Aeolian-derived Iron Major source of iron How much of the iron is soluble? - 1-30% Active area of research: differences by provenance,dust type, processing in cloud, wet vs. dry deposition, chemistry of surface waters Flux: 0.2-1.2 X 10 10 mol y -1 (assuming 2% solubility) Annual Fe flux in dust (mg Fe m -2 y -1 ) Mahowald et al. (2003)

19 Iron Speciation : Complexation

20 Inorganic iron: Fe 2+, Fe 3+, Fe(OH) 3 –Since ocean is oxidizing medium, reduced iron (Fe 2+ ) concentrations are low. –Most Fe 2+ produced by photochemistry, has a short lifetime 99% of Fe found bound to organic ligands –Increases solubility of iron in water column –Thought to be siderophore-type compounds produced by bacteria. –Not available for uptake by eukaryotic phytoplankton

21 Complexation: Active areas of research What is the structure of the ligand? -messy organic molecular structure How do organisms produce it? -current research suggest marine bacteria produce the ligands. How do organisms utilize FeL? -Light breaks down FeL so organisms can grab the Fe’ - Some bacteria and cyanobacteria seem to have uptake mechanisms that will dissociate Fe from siderophore- type ligands. Barbeau et al. (2004)

22 Forms of Iron Dissolved iron: <0.02 µm Colloidal: 0.02-0.4 µm Particulate: >0.4 µm Active area of research: Role of colloidal matter Data from Wu and Boyle, 10N (Atlantic) dissolved + colloidal dissolved

23 Phytoplankton Uptake of Iron Oceanic Coastal Oceanic species have higher growth rates at lower [Fe] They have adapted Their Fe requirement is lower (small Fe:C ratio) Oceanic species are smaller, so they have higher surface area:volume ratio CoastalOceanic Sunda and Huntsman (1995) Fe’

24 µmµmPrasinophyceae : 2 Synechococcus : 1 Prochlorococcus : 0.5 Bolidophyceae : 1.5 µmµm µmµm µmµm micro- pico- …

25 Synecococcus sp. - a very abundant marine photosynthetic bacterium

26 Putting it all together biological loop dissolved Fe (< 0.4  m) biogenic export lateral transport and mixing DUST refractory dust upwelling and vertical mixing mixed layer bottom surface remineralization sediment-water interface lateral transport scavenging & desorption mixing sedimentary deposition scavenging & desorption Fe’ + L’  FeL Developing mathematical model to understand the various processes affecting Fe

27 Model Results: Iron Surface 1000 m Observations Model Parekh et al. (2004)

28 Link between dust flux and CO 2 ? Age (kyr) Dust Flux (mg m -2 yr -1 ) Atmospheric CO 2 (ppm) Figure from Gruber from Martin (1990) +dust  +Fe  +bio. Productivity  +Export  +CO 2 drawdown

29 Marine Physical Chemistry

30 Atmospheric CO 2 Sensitivity to Increased Dust Flux ‘Paleo’ dust estimate from Mahowald et al. (1999) Dust flux greater 5.5 times globally LGM dust flux Present dust flux

31 Macro Nutrients Export of Organic Matter Changes in Biogeochemical Cycling

32 Model result The effect of additional Fe is quite small. ~11 ppm ΔpCO 2 (Pre-industrial -LGM) =80 ppm

33 Iron Fertilization Adding Fe artificially to transfer CO 2 from atmosphere to the sea Open questions: - How effective will it be? - Effect on marine ecology?

34 End